(1) Technical Field
The subject invention is directed toward an electromagnetic clutch having an elastomeric torque cushion disposed between a drive shaft and an armature plate and includes a cylindrical reinforcing member disposed within the torque cushion for preventing the torque cushion from buckling as the cushion is subjected to torque when the clutch is engaged.
(2) Description of the Prior Art
Electromagnetic clutches are commonly used in conjunction with refrigerant compressors in automotive air conditioning applications to translate torque from the power take-off of the automotive engine to a drive shaft, which, in turn, actuates the compressor. More specifically, when the electromagnetic clutch is energized, a magnetic flux flowing through the armature plate pulls the armature plate across a small gap of predetermined width and against the friction plate of a continuously driven pulley to initiate rotation of the drive shaft. The moment at which the armature plate is engaged causes a significant instantaneous increase in the torsional stress acting on the drive shaft. Extreme and repeated increases in the torsional stress acting on the drive shaft contribute to premature failure.
In order to support the armature plate in parallel spaced relation from the friction plate of the rotating pulley and to allow the armature plate to bridge the gap therebetween to engage the friction plate, the prior art teaches the use of springs which interconnect the armature plate and a support structure which is fixed to the drive shaft. When designed for the specific purpose, certain connecting springs known in the prior art are also employed to absorb and therefore reduce the high torsional stresses generated upon engagement between the armature and friction plates. Examples of electromagnetic clutches including arcuately shaped torsional stress dampening connecting springs can be found at U.S. Pat. No. 4,296,851 issued to Pierce on Oct. 27, 1981 entitled Drive Hub with Curved Springs and Drive Keys For Electromagnetic Clutch; U.S. Pat. No. 4,616,742 issued to Matsushita on Oct. 14, 1986 entitled Spring Coupling For An Electromagnetic Clutch; and U.S. Pat. No. 4,972,932 issued to Nakamura et al. on Nov. 27, 1990 entitled Spring Connected Armature Assembly for Electromagnetic Clutch.
In addition to generating high torsional stresses, the operation of refrigerant compressors in these automotive applications as well as the actuation of the electromagnetic clutch produce vibrations which can result in objectional noise and rapid wear and thus concomitant premature failure of the effected parts. In addition, at certain compressor RPMs, the vibration generated can approach the resonant frequency of the compressor's system resulting in resonance and destructive forces in the system.
In view of the above, it is an important design objective to prevent the frequency of the vibration generated at all operating compressor RPMs from approaching the resonant frequency of the air conditioning compressor system. To overcome this problem, and at the same time deal with torsional stresses generated by the actuation of the electromagnetic clutch, the prior art teaches the use of an elastomeric, e.g. rubber, torque cushion disposed between and interconnecting the drive shaft and the armature plate. The torque cushion absorbs torsional stresses upon engagement of the armature plate with the pulley friction plate and protects the drive shaft from frequent loading at extremely high torsional stresses. Examples of electromagnetic clutches employing elastomeric torque cushions can be found at U.S. Pat. No. 3,384,213 issued to Bernard et al. on May 21, 1968 and entitled Electromagnetic Clutch With Carbon Core; U.S. Pat. No. 4,624,354 issued to Koitabashi on Nov. 25, 1986 and entitled Electromagnetic clutch; and U.S. Pat. No. 4,828,090 issued to Matsushita on May 9, 1989 and entitled Electromagnetic Clutch.
Despite these advancements in the art, problems still remain. It has been found that special care must be taken to design the elastomeric torque cushion such that the natural frequency of the compressor/clutch system can only be approached at RPMs below the operating RPM of the system. This is achieved by decreasing the torsional stiffness of the torque cushion which essentially makes the cushion "softer" and also lowers the peak to peak torque response of the system.
On the other hand, the maximum torsional shear stress on the torque cushion must be kept below allowable limits. However, where torsional stiffness is too low, the torque cushion can buckle under the stress generated during the operation of the compressor which ultimately will lead to a failure oft he cushion. Attempts to strengthen the torque cushion in response to stress and vibration include the addition of flat radially extending spring-like rings imbedded in the torque cushion as shown for example at 135 in FIG. 1 of the Bernard '213 patent. Unfortunately, such rings in the prior art have not adequately addressed the problem of buckling. Furthermore, the flat, annular, radially extending springs of the prior art inhibit axial movement of the cushion which is necessary when the armature plate is drawn across the gap to engage the pulley friction plate. After numerous cycles, these flat springs can become plastically deformed which can skew the parallelly mounted armature plate so as to unacceptably bridge the gap between it and the pulley friction plate and/or cause uneven wear.
The subject invention overcomes these problems in an electromagnetic clutch having a torque cushion with a relatively low torsional stiffness such that the natural frequency of the compressor system is below the operating RPMs of the system but which can withstand the maximum sheer stress generated by the system without buckling. Furthermore, the torque cushion employed by the clutch of the subject invention is specifically adapted to maintain the armature plate in a spaced parallel orientation with respect to the pulley friction plate throughout the life of the clutch.